1. Field of the Invention
The present invention is related to a method and an apparatus for embedding information into an image represented by multi-gradation, and related to a method and an apparatus for detecting information embedded in an image, and also related to a storage medium of an image processing program.
2. Description of the Related Art
Very recently, digital watermarking techniques by which electronic information is added in such a format that this electronic information cannot be visibly recognized by human eyes at a glance have been actively researched/developed. Then, these digital watermarking techniques have been utilized by way of various modes. However, the major digital watermarking techniques have been employed via electronic media, whereas substantially no digital watermarking technique has been used via non-electronic media such as paper. This reason is given as follows. That is, when image data stored in a computer is printed out and thereafter the printed image is scanned to be inputted into the computer, since the image data accepts various image converting operations, there is such a problem that it is practically difficult to detect information from the image data under stable condition while a deterioration of image qualities is suppressed. Concretely speaking, the above-described various image converting operations involve D/A and A/D converting operations by printing and scanning operations; color converting operations; binary processing operations by screening; resolution converting operations required due to differences in resolution of printers and scanners; noise produced during scanning operations in addition to skew; irregular shifts (fluctuations within plane) of pixel positions which are caused by mechanical operations of printers and scanners etc. Also, in the case that a digital camera is employed as an image input device, there is another problem of aberration. However, since precision as to printers, scanners, digital cameras and the like is increased, necessities of digital watermarking techniques are increased which can be applied to printing fields, although these problems are still left.
As a technical trial capable of partially solving such a problem, for example, in accordance with a conventional technique described in JP-A-Hei. 11-284833, additional information is expressed by a direction of pixels which have been painted over within a rectangular area formed by a plurality of pixels. However, in this conventional technique, since the rectangular area formed by the plural pixels is employed in order to represent a 1-bit signal, if an amount of information which is embedded into an entire image is increased, then there are such problems that the size of the rectangular area is decreased and the gradation characteristic is decreased. In addition, the following practical difficulty may be readily supposed. That is, since no specific measure has been taken in order to detect the rectangular area from a printed image, it is practically difficult to actually detect the rectangular area so as to identify the additional information.
Also, in another conventional technique described in JP-A-2001-103281, in such a case that a dither method is employed in a halftone process operation of a printer, an image is binary-processed in the unit of an (n×m) dither matrix, and then, dots are added to four corners of the (n×m) pixels corresponding to this binary-processed image so as to represent additional information. This conventional technique owns such a problem that since the additional information is added in the halftone process operation, this technical idea cannot be applied to another printer capable of executing another halftone process operation different from the above-explained halftone process operation.
Also, any of the above-described two publications (JP-A-Hei. 11-284833, and JP-A-2001-103281) does not consider such a case that resolution achieved during the scanning operation is different from resolution achieved during the embedding operation. As a result, for instance, as to an image which has been enlarged, or reduced after being printed, there is such a problem that additional information cannot be acquired from this printed image.
Also, for instance, both JP-A-2001-119558 and JP-A-2001-119562 disclose both a method for embedding a registration signal so as to detect an enlarging/reducing ratio by utilizing a Fourier transformation with respect to a specific image block, and also a method for embedding additional information in such a manner that a block to be embedded is determined based upon a positional relation of a two-dimensional mask, and then, pattern data is added/subtracted with respect to this determined block. Concretely speaking, a signal having a specific frequency (registration signal) is added to a specific block. Then, the block to which the registration signal has been added is detected from, for instance, an image to which a resolution converting operation has been performed. A detection is made that the frequency of the registration signal has been converted into another frequency, and then, a ratio of the resolution converting operation is obtained from a ratio of these frequencies. In accordance with this conventional technique, even when a geometrical converting operation such as the resolution converting operation is carried out, this converting operation is judged, so that the embedding information can be detected.
However, in this method, first of all, the block into which the registration has been embedded must be found out. It is difficult to find out such a block corresponding to the original block into which the registration signal has been embedded from the image whose resolution has been converted. Also, even if such a block could be detected, the following problem occurs. That is, in order to detect the frequency of the registration signal from the block which has been detected, if the registration signal is not embedded at a certain strength, then the frequency of this registration signal can be hardly detected. If the registration signal is embedded at a certain strength into a predetermined specific block, then a difference in the characteristics of this block becomes conspicuous.
Also, as the method of embedding the additional information, in this method, the additional information is embedded in such a way that a certain pattern is added, or subtracted only to a specific block position based upon two-dimensional mask information. However, if the additional information is easily detectable, then the additional information is strongly embedded to the specific block position. As a result, a difference between this specific block position and the portion where the additional information is not embedded may become conspicuous. Conversely, if the additional information is weakly embedded to the specific block position, then the additional information can be hardly detected. Also, in such a case that the block size used when the additional information is embedded is changed, the following problem may also occur. That is, if this change of the block sizes is not precisely notified, then the additional signal cannot be decoded. For example, in the case that an image output device such as a printer capable of performing a halftone process operation is used, if additional information is embedded based upon such a block size which is excessively smaller than a cell size of the halftone process operation, then substantially no information is left during printing operation. As a result, the block size must be made large substantially equal to the cell size of the halftone process operation. Also, in the case that irregular dot shift occurred within a plane are large, and also positional precision is slightly lowered, such an adjustment that a block size is increased so as to easily detect additional information may be effectively employed. However, in such a case that if the block size is not known on the detection side, then the additional signal cannot be decoded, the management as to the devices provided on the detection side becomes complex.
Furthermore, as to measures with respect to degradation in image qualities, since only an amplitude control of an embedding pattern is merely prepared, if a block size is made large, then a detection ratio of the embedding pattern is increased. However, no measure is taken to such a fact that the resulting image quality is degraded. As a consequence, there is only such a measure that such additional information with a small amplitude is embedded, by which the degradation of the image quality do not become conspicuous, so that this additional information can be hardly detected. In order to compensate for this difficult detection of the additional information, a small amount of information maybe repeatedly embedded into an entire image, and then, this embedded information may be recovered by using a statistical process operation. However, the recovery steps become complex, and the embedding information amount is decreased. Furthermore, since both the registration signal used to detect the enlarging ratio and the information to be transferred are embedded by using the different embedding methods, another problem may occur. That is, the registration signal may interfere with the information to be transferred.